Ceramic substrate and process for producing the same

ABSTRACT

It is a an object of the present invention to provide a ceramic substrate for a semiconductor producing/examining device which has high fracture toughness value, exellent thermal shock resistivity, high thermal conductivity and an excellent temperature rising and falling properties, and is preferable as a hot plate, an electrostatic chuck, a wafer prober and the like. A ceramic substrate, for a semiconductor producing/examining device, having a conductor formed inside thereof or on the surface thereof of the present invention is the ceramic substrate, wherein said ceramic substrate has been sintered such that a fractured section thereof exhibits intergranular fracture.

TECHNICAL FIELD

The present invention relates to a ceramic substrate used mainly in thesemiconductor industry, and particularly to a ceramic substrate that issuitable for a hot plate, an electrostatic chuck, a wafer prober and thelike, and is superior in thermal shock resistance, heat uniformity,chuck power and so on.

BACKGROUND ART

Semiconductors are very important goods necessary in various industries.A semiconductor chip is produced, for example, by slicing a siliconmonocrystal into a predetermined thickness to produce a silicon wafer,and then forming a plurality of integrated circuits and the like on thissilicon wafer.

In the process for producing this semiconductor chip, a silicon waferput on an electrostatic chuck is subjected to various treatments such asetching and CVD to form a conductor circuit, an element and the like. Atthis time, corrosive gas such as gas for deposition or gas for etchingis used; therefore, it is necessary to protect an electrostaticelectrode layer from corrosion by these gas. Since it is also necessaryto induce adsorption power, the electrostatic electrode layer is usuallycoated with a ceramic dielectric film and the like.

As this ceramic substrate, Japanese Patent gazette No. 279850 andJapanese Patent gazette No. 2513995, JP Kokai Hei 11-74064 and so ondescribe electrostatic chucks with a heater which is produced throughthe process of stacking the green sheets, on which paste of a metal suchas tungsten (W) is printed.

SUMMARY OF THE INVENTION

However, in ceramic substrates produced by such a process, problems asfollows arise: cracks start to be generated at edges of their conductorlayer such as a resistance heating element when thermal shock is givento the substrates; or a high temperature area is generated along theresistance heating element.

Furthermore, a scatter is generated in chuck power so that sufficientadsorption power cannot be obtained.

About an internal electrode or a resistance heating element, a leakagecurrent between the electrodes or between the resistance heatingelements at high temperature comes into a problem.

The inventors made eager investigations to solve the above-mentionedproblems. As a result, the inventors have newly found out that theseproblems can be solved by making the edge of a cross section ofconductor layer constituting an electrostatic electrode, an RF electrodeand a resistance heating element into a peaked shape. Thus, the presentinvention has been made.

Namely, the present invention is a ceramic substrate comprising aconductor layer formed therein,

characterized in that a section of the edge of the conductor layer is ina peaked shape.

In the case that the above-mentioned conductor layer in the ceramicsubstrate of the present invention is a resistance heating element, thepresent invention functions as a hot plate.

In the case that the above-mentioned conductor layer is an electrostaticelectrode, the present invention functions as an electrostatic chuck.

In the ceramic substrate, the conductor layer desirably has a portion inthe peaked-shape having a width of 0.1 to 200 μm.

The process for producing a ceramic substrate of the present inventionis characterized by printing a conductor layer on a ceramic green sheet,integrating the green sheet with another green sheet under heating andpressure, and then sintering the ceramic powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plain view that schematically illustrates one example of aceramic heater using a ceramic substrate of the present invention.

FIG. 2 is a partially enlarged sectional view of the ceramic heaterillustrated in FIG. 1;

FIG. 3 is a sectional view that schematically illustrates one example ofan electrostatic chuck using a ceramic substrate of the presentinvention.

FIG. 4 is a sectional view taken along A—A line of the ceramic heaterillustrated in FIG. 3.

FIG. 5 is a sectional view that schematically illustrates one example ofan electrostatic chuck using a ceramic substrate of the presentinvention.

FIG. 6 is a sectional view that schematically illustrates one example ofan electrostatic chuck using a ceramic substrate of the presentinvention.

FIG. 7 is a sectional view that schematically illustrates one example ofan electrostatic chuck using a ceramic substrate of the presentinvention.

FIGS. 8(a) through (d) are sectional views that schematically illustratea part of the process for producing the electrostatic chuck illustratedin FIG. 5.

FIG. 9 is a horizontal sectional view that schematically illustrates ashape of an electrostatic electrode constituting an electrostatic chuckaccording to the present invention.

FIG. 10 is a horizontal sectional view that schematically illustrates ashape of an electrostatic electrode constituting an electrostatic chuckaccording to the present invention.

FIG. 11 is a sectional view that schematically illustrates the statethat an electrostatic chuck according to the present invention is fittedinto a supporting case.

FIG. 12 is a sectional view that schematically illustrates a waferprober using a ceramic substrate of the present invention.

FIG. 13 is a sectional view that schematically illustrates a guardelectrode of the wafer prober illustrated in FIG. 12.

FIG. 14 is an SEM photograph showing a destruction section of a ceramicsubstrate constituting the electrostatic chuck according to Example 1.

EXPLANATION OF SYMBOLS

1, 11, 63 ceramic substrate

2, 22, 32 a, 32 b chuck positive electrode electrostatic layer

3, 23, 33 a, 33 b chuck negative electrode electrostatic layer

2 a, 3 a semicircular part

2 b, 3 b comb-teeth-shaped part

4 ceramic dielectric film

5, 12, 25, 61 resistance heating element

6, 13, 18 external terminal

7 metal line

8 Peltier device

9 silicon wafer

10 ceramic heater

14 bottomed hole

15 through hole

16, 17, 18 plated through hole

20, 30, 101, 201, 301, 401 electrostatic chuck

25 a metal covering layer

35, 36 blind hole

41 supporting case

42 coolant outlet

43 inhalation duct

44 coolant inlet

45 heat insulator

62 chuck top conductor layer

65 guard electrode

66 ground electrode

66 a non-electrode forming area

67 groove

68 suction hole

501 wafer prober

DETAILED DISCLOSURE OF THE INVENTION

The present invention is a ceramic substrate comprising a conductorlayer formed therein,

-   -   characterized in that a section of the edge of the conductor        layer is in a peaked shape.

In the case that the conductor layer in the above-mentioned ceramicsubstrate is a resistance heating element, the present inventionfunctions as a hot plate. In the case that the conductor layer is anelectrostatic electrode, the present invention functions as anelectrostatic chuck.

The ceramic substrate of the present invention is desirably used at 150°C. or higher, and is most desirably used at 200° C. or higher.

In the ceramic substrate, the conductor layer desirably has a portion inthe peaked shape having a width of 0.1 to 200 μm, and more desirably hasthe peaked portion having a width of 5 to 100 μm.

In the process for producing the above-mentioned ceramic substrate, thefollowing process is desirably adopted: a process of printing aconductor layer on a ceramic green sheet, integrating the green sheetwith another green sheet under heating and pressure, and then sinteringthe ceramic powder.

In the case that the conductor layer embedded in the ceramic substrateused in the present invention is an electrode for inducing chuck powerof an electrostatic chuck, that is, an electrostatic electrode, theedges of the electrode is in a peaked shape so that an electric fieldconcentrates along the edge. Thus, large chuck power is considered to beinduced.

The reason why cracks are generated by thermal shock is analyzed. Aconductor layer produced by a conventional printing process and the likehas a substantially rectangular section and, therefore, has a faceperpendicular to a wafer processing surface (a surface for heating,supporting or adsorbing a semiconductor wafer). Moreover, the thermalexpansion coefficient of the conductor layer is different from that ofthe ceramic constituting the ceramic substrate. Therefore, if theceramic substrate is heated or cooled, power for separating the faceperpendicular to the wafer processing surface of the conductor layerfrom the ceramic around the face is produced. This power would causegeneration of cracks easily.

However, in the ceramic substrate of the present invention, itsconductor layer has no face perpendicular to the wafer processingsurface. As a result, cracks are not easily generated.

The reason why a high-temperature area is generated along the resistanceheating element would be that heat is easily accumulated in thefollowing portion inside the resistance heating element: the lineintersection portion between the face perpendicular to the waferprocessing surface and the face horizontal to the wafer processingsurface. The reason why such a heat-accumulating phenomenon is caused isunclear. It is however presumed that heats are emitted and transmittedfrom both of the face perpendicular to the wafer processing surface andthe face horizontal to the wafer processing surface so that these heatscross each other in the line intersection portion.

However, the ceramic substrate of the present invention has no faceperpendicular to the wafer processing surface. Therefore, such a heataccumulating phenomenon is not caused so that heat uniformity of thewafer processing surface is superior.

Furthermore, in the case that the ceramic substrate has therein aconductor layer such as an electrode (a guard electrode and a groundelectrode of a wafer prober an electrode of an electrostatic chuck, anRF electrode and the like) or a resistance heating element, the facesperpendicular to the wafer processing surface of the conductor layer, ifexist, confront each other. Thus, a leakage current is easily generatedat high temperature. In the present invention, a section of the edge ofthe conductor is in a peaked shape. No faces confront each other. Thus,a leakage current is not easily generated at high temperature.

The ceramic substrate of the present invention is used in the range of150° C. or higher, and preferably in the range of 200° C. or higher.

The conductor layer of the present invention may be an electrostaticelectrode, a resistance heating element, an RF electrode and also may bea guard electrode or a ground electrode used in a wafer prober.

The conductor layer desirably has the peaked shape portion having awidth of 0.1 to 200 μm. If the width of the peaked shape portion is over200 μm, its ohmic resistance value is scattered. On the other hand, ifthe width is below 0.1 μm, the effect of preventing cracks is notcaused. The width of the peaked shape portion is optimally from 5 to 100μm. The curvature radius of the peaked shape portion is optimally from0.5 to 500 μm.

In the ceramic substrate of the present invention, the pore diameter ofthe maximum pore is desirably 50 μm or less. The porosity thereof isdesirably 5% or less. It is also desirable that no pores are present or,if present, the pore diameter of the maximum pore is 50 μm or less.

If no pores are present, breakdown voltage is especially high at hightemperature. Conversely, if pores are present, fracture toughnessbecomes high. Thus, which is designed depends on required properties.

The reason why fracture toughness becomes high on the basis of thepresence of pores is unclear, but it is presumed that the reason isbased on stop of development of cracks by the pores.

The reason why the pore diameter of the maximum pore is desirably 50 μmor less in the present invention is that if the pore diameter is over 50μm, it is difficult to keep breakdown voltage property at hightemperature and particularly at 200° C. or higher.

The pore diameter of the maximum pore is desirably 10 μm or less. Thisis because the amount of warp becomes small at 200° C. or higher.

The porosity and the pore diameter of the maximum pore are adjusted bypress time, pressure and temperature at the time of sintering, oradditives such as SiC and BN. Since SiC or BN hinders sintering, porescan be produced.

When the pore diameter of the maximum pore is measured, 5 samples areprepared. The surfaces thereof are polished into mirror planes. With anelectron microscope, ten points on the surface are photographed with2000 to 5000 magnifications. The maximum pore diameter is selected fromthe photos obtained by the photographing, and the average of the 50shoots is defined as the pore diameter of the maximum pore.

The porosity is measured by Archimedes' method. This is a method ofcrushing a sintered product to pieces, putting the pieces into anorganic solvent or mercury to measure the volume thereof, obtain thetrue specific gravity of the pieces from the weight and the volumethereof, and calculating the porosity from the true specific gravity andapparent specific gravity.

The diameter of the ceramic substrate of the present invention isdesirably 200 mm or more. It is especially desirable that the diameteris 12 inches (300 mm) or more. This is because semiconductor wafershaving such a diameter become main currents of the next-generationsemiconductor wafers.

The thickness of the ceramic substrate of the present invention isdesirably 50 mm or less, and especially desirably 25 mm or less.

If the thickness of the ceramic substrate is over 25 mm, the thermalcapacity of the ceramic substrate is too large. Particularly when atemperature control means is set up to heat or cool the substrate,temperature-following property may become poor because of the largethermal capacity.

The thickness of the ceramic substrate is optimally 5 mm or less.Incidentally the thickness of the ceramic substrate is desirably 1 mm ormore.

The ceramic material constituting the ceramic substrate of the presentinvention is not especially limited. Examples thereof include nitrideceramics, carbide ceramics, and oxide ceramics.

Examples of the nitride ceramics include metal nitride ceramics such asaluminum nitride, silicon nitride and boron nitride.

Examples of the carbide ceramics include metal carbide ceramics such assilicon carbide, zirconium carbide, tantalum carbide, and tungstencarbide.

Examples of the oxide ceramics include metal oxide ceramics such asalumina, zirconia, cordierite and mullite.

These ceramics may be used alone or in combination of two or morethereof.

Among these ceramics, the nitride ceramics and oxide ceramics arepreferred.

Aluminum nitride is most preferred among nitride ceramics since itsthermal conductivity is highest, that is, 180 W/m·K.

The ceramic substrate preferably contains 0.1 to 5% by weight of oxygen.In this case, for example, the sintering of the nitride ceramic advanceseasily. Thus, even if the nitride ceramic contains pores, the pores areindependently of each other so that the breakdown voltage is improved.

If the content is below 0.1% by weight, the breakdown voltage cannot bekept. Contrarily, if the content is over 5% by weight, the breakdownvoltage property at high temperature of oxides becomes poor so that thebreakdown voltage drops as well. If the oxygen content is over 5% byweight, the thermal conductivity drops so that temperature-rising andtemperature-fall property becomes poor.

In order to incorporate oxygen into, for example, the nitride ceramic,it is general that the nitride ceramic powder is fired in an oxidizingatmosphere or that a metal oxide is mixed with ingredient powder of thenitride ceramic and then the mixture is sintered. In the case of theoxide ceramic, another oxide is mixed with it to make a complex oxide.

Examples of the metal oxide include yttria (Y₂O₃), alumina (Al₂O₃),rubidium oxide (Rb₂O), lithium oxide (Li2O) and calcium oxide (CaCO₃).

The content by percentage of these metal oxides is preferably from 0.1to 20% by weight.

In the present invention, the ceramic substrate preferably contains 5 to5000 ppm of carbon.

The ceramic substrate can be blackened by incorporating carbon. Thus,when the substrate is used as a heater, radiant heat can be sufficientlyused.

Carbon may be amorphous or crystalline. When amorphous carbon is used, adrop in the volume resistivity at high temperature can be prevented.When crystalline carbon is used, a drop in the thermal conductivity athigh temperature can be prevented. Therefore, crystalline carbon andamorphous carbon may be used together dependently on use. The carboncontent is preferably from 50 to 2000 ppm.

When carbon is incorporated into the ceramic carbon, carbon ispreferably incorporated in the manner that its brightness will be belowN4 as a value based on the rule of JIS Z 8721. The ceramic having such abrightness is superior in radiant heat capacity and covering property.

The brightness N is defined as follows: the brightness of ideal black ismade to 0; that of ideal white is made to 10; respective colors aredivided into 10 parts in the manner that the brightness of therespective colors is recognized stepwise between the brightness of blackand that of white; and the resultant parts are indicated by symbols N0through N10, respectively.

Actual brightness is measured by comparison with color signalscorresponding to N0 through N10. One place of decimals in this case ismade to 0 or 5.

The ceramic substrate of the present invention is a ceramic substrateused in a device for producing a semiconductor or examining asemiconductor. Examples of specific devices include an electrostaticchuck, a hot plate (ceramic heater) and a wafer prober.

In the case that the conductor formed inside the ceramic substrate is aresistance heating element, the ceramic substrate can be used as aceramic heater (hot plate).

FIG. 1 is a plain view that schematically shows an example of a ceramicheater that is one embodiment of the ceramic substrate of the presentinvention. FIG. 2 is a partially enlarged section showing a part of theceramic heater shown in FIG. 1.

A ceramic substrate 11 is made in a disk form. Inside the ceramicsubstrate 11, resistance heating elements 12 as temperature controlmeans are formed in the pattern of concentric circles. About theseresistor heating elements 12, two concentric circles near to each otherare connected to produce one line as a circuit, and external terminals13 that become inputting/outputting terminal pins are connected to bothends of the circuit via plated through holes 19. As shown in FIG. 2,sections of both edges of the resistance heating element 12 are in apeaked shape. Therefore, cracks based on thermal shock and the like arenot easily generated in the ceramic substrate 11, and aheat-accumulating phenomenon is not generated in the edge of theresistance heating elements 12, either. Moreover, distribution oftemperature in the wafer processing surface is not caused so that thetemperature of the face becomes uniform.

As shown in FIG. 2, through holes 15 are made in the ceramic substrate11, and support pins 26 are inserted into the through holes 15 tosupport a silicon wafer 9. By moving the support pins 26 up and down, itis possible to receive the silicon wafer 9 from a carrier machine, putthe silicon wafer 9 on a wafer processing surface 11 a of the ceramicsubstrate 11 and heat the wafer 9, or support the silicon wafer 9 in thestate that the wafer 9 is apart from the wafer processing surface 11 aat an distance of about 50 to 2000 μm and then heat the wafer 9.Bottomed holes 14 into which temperature-measuring elements such asthermocouples are fitted are made in a bottom surface 11 a of theceramic substrate 11. When an electric current is passed through theresistance heating elements 12, the ceramic substrate 11 is heated sothat a product to be heated, such as a silicon wafer, can be uniformlyheated.

In the case of a ceramic heater, the resistance heating elements whoseedge has a peaked-shape section are set inside the ceramic substrate. Inthis manner, the resistance heating elements have the advantages of thepresent invention. In the case of an electrostatic chuck or wafer proberthat will be described later, the resistance heating elements may be seton the bottom surface of the ceramic substrate. In this case, a sectionof the edge of an electrostatic electrode, a guard electrode, a groundelectrode and so on is made into a peaked shape. In this manner, theseelectrodes have the above-mentioned advantageous effects of the presentinvention.

In the case that the resistance heating elements are set inside theceramic substrate, an coolant outlet for a coolant such as air as acooling means may be made in a supporting case into which the ceramicsubstrate is fitted. In the case that the resistance heating elementsare set inside the ceramic substrate, each of the resistance heatingelements may be made of a plurality of layers. In this case, thepatterns of the respective layers may be formed to complement themmutually. A pattern is desirably formed on any one of the layers whenbeing viewed from the heating surface. For example, a structure having astaggered arrangement is desirable.

The resistance heating element is desirably made of a metal such as anoble metal (gold, silver, platinum and palladium), lead, tungsten,molybdenum, nickel; and a conductive ceramics such as tungsten carbideand molybdenum carbide. These can make the ohmic resistance value highand make the thickness itself large to prevent the element-disconnectionand the like, and are not easily oxidized. Their thermal conductivity isnot easily lowered, either. These may be used alone or in combination oftwo or more.

Since it is necessary that the temperature of the whole of the ceramicsubstrate is made uniform, the resistance heating element preferably hasa pattern of concentric circles shown in FIG. 1, or a combination of apattern of concentric circles and that of bent lines. The thickness ofthe resistance heating element is desirably from 1 to 50 μm, and thewidth thereof is preferably from 5 to 20 mm. The resistance heatingelement desirably has the peaked shape portion having a width of 0.1 to200 μm.

By changing the thickness or the width of the resistance heatingelement, its ohmic resistance value can be changed; however, theabove-mentioned ranges are most practical. As the resistance heatingelement becomes thinner or narrower, the ohmic resistance value of theresistance heating element becomes larger.

If the resistance heating element is set inside, the distance between aheating surface and the resistance heating element becomes close so thatuniformity of the temperature of the surface becomes poor. Thus, it isnecessary to widen the width of the resistance heating element itself.Since the resistance heating element is set inside the ceramicsubstrate, it is unnecessary to consider adhesiveness of this element tothe ceramic substrate.

A section of the resistance heating element may be rectangular,elliptic, spindle-shaped, or semicylindrical. The section is desirablyflat. A section of the edge of the resistance heating element isdesirably in a peaked shape. This is because; if the section is flat,heat is easier to be radiated toward the heating surface, so that theamount of heat transmitted to the heating surface can be made large.Thus, temperature distribution in the heating surface is not easilyformed. The shape of the resistance heating element may be spiral.

When the resistance heating elements are formed inside the ceramicsubstrate, the resistance heating elements are desirably formed withinthe domain extending from the bottom of the substrate up to 60% of thethickness thereof. This is because the temperature distribution in theheating surface is removed so that a semiconductor wafer can beuniformly heated.

A thing to be heated, such as a semiconductor wafer, can be directly puton the heating surface and then heated. The product may be kept about 50to 200 μm apart from the heating surface and then heated.

In order to form the resistance heating elements on the bottom surfaceof the ceramic substrate for semiconductor devices of the presentinvention, or inside the ceramic substrate, it is preferred to use aconductor containing paste comprising a metal or a conductive ceramic.

Namely, in the case that the resistance heating elements are formed onthe bottom surface of the ceramic substrate, sintering is usuallyperformed to produce the ceramic substrate and then a layer of theconductor containing paste is formed on the surface of the ceramicsubstrate and sintered to produce the resistance heating elements.

On the other hand, in the case that the resistance heating elements 12are formed inside the ceramic substrate 12 as shown in FIGS. 1 and 2, alayer of the conductor containing paste is formed on a green sheet andthen the green sheet is integrated with another green sheet underheating and pressure to produce a lamination of the green sheets. Atthis time, the conductor containing paste layer having the edge in apeaked shape can be formed by heating the lamination up to a temperatureat which the conductor containing paste after drying is liable to deformto some extent. Thereafter, the lamination is sintered to make theresistance heating elements whose edge has a section in a peaked shapeinside the ceramic substrate.

The conductor containing paste is not limited. Preferably, the conductorcontaining paste contains a resin, a solvent, a thickener and so on, aswell as metal particles or conductive ceramic particles to ensureconductivity.

The material of the metal particles or the conductive ceramic particlesmay be the above-mentioned ones. The particle diameter of the metalparticles or the conductive ceramic particles is preferably from 0.1 to100 μm. If the diameter is too fine, that is, below 0.1 μm, theseparticles are easily oxidized. On the other hand, if the diameter isover 100 μm, these particles are not easily sintered so that the ohmicresistance value becomes large.

The shape of the metal particles may be spherical or scaly. In the casethat these metal particles are used, a mixture of spherical particlesand scaly particles can be used.

The case that the above-mentioned metal particles are scaly, or are amixture of spherical and scaly particles is profitable since metaloxides are easily kept between the metal particles so that the adhesionbetween the resistance heating elements and ceramic substrate is madesure and the ohmic resistance value can be made large.

Examples of the resin used in the conductor containing paste includeacrylic resins, epoxy resins, and phenol resins. An example of thesolvent is isopropyl alcohol and the like. An example of the thickeneris cellulose and the like.

When the conductor containing paste for the resistance heating elementsis formed on the surface of the ceramic substrate, it is preferred toadd not only the metal particles but also a metal oxide to the conductorcontaining paste and sinter the metal particles and the metal oxide. Bysintering the metal oxide together with the metal particles as describedabove, the ceramic substrate still more adhered to the metal particlescan be obtained.

The reason why the adhesiveness of the metal particles to the ceramicsubstrate is improved by the mixing of the metal oxide is unclear, butwould be as follows: the surface of the metal particles or the surfaceof the ceramic substrate comprising non-oxides is slightly oxidized toform oxidized films; and the oxidized films are sintered and integratedwith each other through the metal oxide, so that the metal particles areclosely adhered to the ceramic. In the case that the ceramicconstituting the ceramic substrate is an oxide, the surface thereof isnaturally made of the oxide. Therefore, the conductor layer superior inadhesiveness is formed.

Preferred examples of the above-mentioned oxide may be at least oneselected from the group consisting of lead oxide, zinc oxide, silica,boron oxide (B₂O₃), alumina, yttria and titania.

These oxides can improve the adhesiveness between the metal particlesand the ceramic substrate without increasing the ohmic resistance valueof the resistance heating element.

When the total amount of the metal oxides is made to 100 parts byweight, the weight ratio of lead oxide, zinc oxide, silica, boron oxide(B₂O₃), alumina, yttria and titania is as follows: lead oxide: 1 to 10,silica: 1 to 30, boron oxide: 5 to 50, zinc oxide: 20 to 70, alumina: 1to 10, yttria: 1 to 50 and titania: 1 to 50, and the ratio preferablybeing adjusted within the scope that the total thereof is not over 100parts by weight.

By adjusting the amounts of these oxides within these ranges, theadhesiveness to the ceramic substrate can be particularly improved.

The addition amount of the metal oxides to the metal particles ispreferably from 0.1% by weight (including 0.1%) to 10% by weight (notincluding 10%). The area resistivity when the conductor containing pastehaving such a structure is used to form the resistance heating elementis preferably from 1 to 45 mΩ/□.

If the area resistivity is over 45 mΩ/□, the calorific value to anapplied voltage becomes too large so that, in the ceramic substrate forsemiconductor devices wherein resistance heating elements are set on itssurface, its calorific value is not easily controlled. If the additionamount of the metal oxides is 10% or more by weight, the arearesistivity exceeds 50 mΩ/□ so that the calorific value becomes toolarge. Thus, temperature-control is not easily performed so that theuniformity in temperature distribution becomes poor.

In the case that the resistance heating elements are formed on thesurface of the ceramic substrate, a metal covering layer is preferablyformed on the surfaces of the resistance heating elements. The metalcovering layer prevents a change in the ohmic resistance value resultingfrom the oxidization of the inner metal sintered product. The thicknessof the formed metal covering layer is preferably from 0.1 to 10 μm.

The metal used for forming the metal covering layer is not particularlylimited provided that the metal is a metal which is hardly oxidized.Specific examples thereof include gold, silver, palladium, platinum, andnickel. These may be used alone or in combination of two or more. Amongthese metals, nickel is preferred.

In the case that the resistance heating elements are formed inside theceramic substrate, no covering is necessary since the surfaces of theresistance heating elements are not oxidized.

In the case that the conductor formed inside the ceramic substrate is anelectrostatic electrode layer, the ceramic substrate can be used as anelectrostatic chuck. In this case, an RF electrode and resistanceheating elements may be formed as conductors below the electrostaticelectrode and inside the ceramic substrate.

FIG. 3 is a vertical sectional view that schematically shows oneembodiment of an electrostatic chuck according to the present invention.FIG. 4 is a sectional view taken on A—A line of the electrostatic chuckshown in FIG. 3.

In this electrostatic chuck 101, an electrostatic electrode layercomposed of a chuck positive electrostatic layer 2 and a chuck negativeelectrostatic layer 3 is buried in a disk-shaped ceramic substrate 1. Athin ceramic layer 4 (hereinafter referred to as a ceramic dielectricfilm) is made on the electrostatic electrode layer. A silicon wafer 9 isput on the electrostatic chuck 101 and is earthed.

As shown in FIG. 4, the chuck positive electrostatic layer 2 is composedof a semicircular part 2 a and a comb-teeth-shaped part 2 b. The chucknegative electrostatic layer 3 is also composed of a semicircular part 3a and a comb-teeth-shaped part 3 b. These chuck positive electrostaticlayer 2 and chuck negative electrostatic layer 3 are arranged by facingeach other so that the comb-teeth-shaped parts 2 b and 3 b cross eachother. The positive side and the negative side of a direct power sourceare connected to the chuck positive electrostatic layer 2 and chucknegative electrostatic layer 3, respectively. Thus, a direct current V₂is applied thereto.

As shown in FIG. 3, a section of the edges of the resistance heatingelements 5, and a section of the edge of the semicircular parts 2 a and3 a and the comb-teeth-shaped parts 2 b and 3 b, which constitute thechuck positive electrostatic layer 2 and the chuck negativeelectrostatic layer 3, respectively, are in a peaked shape.

In order to control the temperature of the silicon wafer 9, resistanceheating elements 5 in the form of concentric circles as viewed from theabove, as shown in FIG. 1, are set up inside the ceramic substrate 1.External terminals are connected and fixed to both ends of theresistance heating elements 5, and a voltage V₁ is applied thereto. Abottomed hole into which a temperature-measuring element is inserted anda through hole through which a support pin (not illustrated) thatsupports the silicon wafer 9 and moves it up and down penetrates aremade in the ceramic substrate 1, as shown in FIGS. 1 and 2 but not shownin FIGS. 3 and 4. The resistance heating elements may be formed on thebottom surface of the ceramic substrate.

When this electrostatic chuck 101 is caused to work, a direct voltage V₂is applied to the chuck positive electrostatic layer 2 and the chucknegative electrostatic layer 3. In this way, the silicon wafer 9 isadsorbed and fixed onto the chuck positive electrostatic layer 2 and thechuck negative electrostatic layer 3 through the ceramic dielectric film4 by electrostatic action of these electrodes. The silicon wafer 9 isfixed onto the electrostatic chuck 101 in this way, and subsequently thesilicon wafer 9 is subjected to various treatments such as CVD.

In the electrostatic chuck 101 according to the present invention, thesection of the edge of the chuck positive and negative electrostaticlayers 2 and 3 is in a peaked shape; therefore, an electric fieldconcentrates along the edge so that a large chuck power is induced.Since the section of the edge of the chuck positive and negativeelectrostatic layers 2 and 3 and the resistance heating elements 5 is ina peaked shape, cracks are not easily generated and no heat-accumulatingphenomenon is generated in the edge of the resistance heating elements5.

In the electrostatic chuck 101, the ceramic dielectric film 4 is made ofa nitride ceramic containing oxygen, and desirably has a porosity of 5%or less and a maximum pore diameter of 50 μm or less. The pores in thisceramic dielectric film 4 are desirably composed of pores independent ofeach other.

In the ceramic dielectric film 4 having such a structure, it does nothappen that gas and so on which will lower the breakdown voltagepenetrate through the ceramic dielectric film to corrode theelectrostatic electrodes. Moreover, the breakdown voltage of the ceramicdielectric film does not drop even at high temperature.

Besides the resistance heating elements 12, a Peltier device (see FIG.7) is mentioned as the temperature control means.

In the case that the Peltier device is used as the temperature controlelement, both heating and cooling can be attained by changing thedirection along which an electric current passes. Thus, this case isadvantageous.

As shown in FIG. 7, the Peltier device 8 is formed by connecting p typeand n type thermoelectric elements 81 in series and then jointing theresultant to a ceramic plate 82 and the like.

Examples of the Peltier device include silicon/germanium,bismuth/antimony, and lead/tellurium materials and the like.

The electrostatic chuck of the present invention has a structure asshown in, for example, FIGS. 3 and 4. The raw materials of the ceramicsubstrate and so on have already been described. The following willdescribe other respective members constituting the electrostatic chuck,and other embodiments of the electrostatic chuck of the presentinvention in detail and successively.

The material of the ceramic dielectric film constituting theelectrostatic chuck of the present invention is not particularlylimited. Examples thereof include oxide ceramics, nitride ceramics andoxide ceramics and the like. Among these, nitride ceramics arepreferred.

As this nitride ceramic, the same as materials of the above-mentionedceramic substrate can be listed up. The nitride ceramic desirablycontains oxygen. In this case, the sintering of the nitride ceramicadvances easily. In the case that the nitride ceramic substrate includespores, the pores are formed to be independent of each other so that thebreakdown voltage is improved.

In order to incorporate oxygen into the nitride ceramic, a metal oxideis usually mixed with ingredient powder of the nitride ceramic, and thenthe mixture is sintered.

The metal oxide may be alumina (Al₂O₃), silicon oxide (SiO₂), and thelike.

The addition amount of such a metal oxide is preferably from 0.1 to 10parts by weight per 100 parts by weight of the nitride ceramic.

By setting the thickness of the ceramic dielectric film to 50 to 5000μm, sufficient breakdown voltage can be kept without lowering chuckingpower.

If the thickness of the ceramic dielectric film is less than 50 μm, thethickness is too thin to obtain sufficient breakdown voltage. Thus, whena silicon wafer is put and adsorbed thereon, the ceramic dielectric filmmay undergo dielectric breakdown. On the other hand, if the thickness ofthe ceramic dielectric film is over 5000 μm, the distance between thesilicon wafer and the electrostatic electrodes is large so that thecapability of adsorbing the silicon wafer is lowered. The thickness ofthe ceramic dielectric film is preferably from 100 to 1500 μm.

Preferably, the porosity of the ceramic dielectric film is 5% or lessand the pore diameter of maximum pores is 50 μm or less.

If the porosity is over 5%, the number of the pores increases and thediameter of the pores becomes too large so that the pores are easilyconnected to each other. In ceramic dielectric films having such astructure, their breakdown voltage drops.

When the pore diameter of the maximum pores is over 50 μm, breakdownvoltage cannot be kept at high temperature even if oxides are present atboundaries between the grains. The porosity is preferably from 0.01 to3%. The diameter of the maximum pores is preferably from 0.1 to 10 μm.

The ceramic dielectric film desirably contains 50 to 5000 ppm of carbon.This is because carbon can hide the electrode pattern arranged in theelectrostatic chuck and give high radiant heat. As the volumeresistivity is lower, the adsorption power of the silicon wafer becomeshigher at low temperatures.

The reason why pores may be present to some extent in the ceramicdielectric film in the present invention is that fracture toughness canbe made high. In this way, thermal shock resistance can be improved.

The electrostatic electrode may be, for example, a sintered product madeof a metal or a conductive ceramic, or a metal foil. As the metalsintered product, a product made of at least one selected from tungstenand molybdenum is preferable. The metal foil is preferably made of thesame as the raw material of the metal sintered product as well. This isbecause these metals are not relatively liable to be oxidized and havesufficient conductivity for electrodes. In this case, the edge of themetal foil is heated and pressed, or is made into a peaked shape bychemical or physical etching. Examples of the chemical etching includeetching with an acidic or alkaline solution. Examples of the physicaletching include ion beam etching, and plasma etching. As the conductiveceramic, there may be used at least one selected from carbides oftungsten and molybdenum.

FIGS. 9 and 10 are horizontal sectional views, each of whichschematically shows an electrostatic electrode in other electrostaticchuck. In an electrostatic chuck 20 shown in FIG. 9, a chuck positiveelectrostatic layer 22 and a chuck negative electrostatic layer 23 in asemicircular form are formed inside a ceramic substrate 1. In anelectrostatic chuck shown in FIG. 10, chuck positive electrostaticlayers 32 a and 32 b and chuck negative electrostatic layers 33 a and 33b, each of which has a shape obtained by dividing a circle into 4 parts,are formed inside a ceramic substrate 1. The two chuck positiveelectrostatic layers 22 a and 22 b and the two chuck negativeelectrostatic layers 33 a and 33 b are formed to cross.

A section of the edge of these electrostatic electrodes is also in apeaked shape. An electric field, therefore, concentrates along the edgeof the electrostatic electrodes so that a large chuck power is induced.Cracks are not easily generated in the ceramic substrate.

In the case that an electrode having a form that an electrode in theshape of a circle and the like is divided is formed, the number ofdivided pieces is not particularly limited and may be 5 or more. Itsshape is not limited to a fan-shape.

Examples of the electrostatic chuck in the present invention include theelectrostatic chuck 101 having a structure wherein the chuck positiveelectrostatic layer 2 and the chuck negative electrostatic layer 3 arearranged between the ceramic substrate 1 and the ceramic dielectric film4 and the resistance heating elements 5 are set up inside the ceramicsubstrate 1, as shown in FIG. 3; the electrostatic chuck 201 having astructure wherein the chuck positive electrostatic layer 2 and the chucknegative electrostatic layer 3 are arranged between the ceramicsubstrate 1 and the ceramic dielectric film 4 and the resistance heatingelements 25 are disposed on the bottom surface of the ceramic substrate1, as shown in FIG. 5; the electrostatic chuck 301 having a structurewherein the chuck positive electrostatic layer 2 and the chuck negativeelectrostatic layer 3 are arranged between the ceramic substrate 1 andthe ceramic dielectric film 4 and the metal line 7, which is aresistance heating element, is buried in the ceramic substrate 1, asshown in FIG. 6; and the electrostatic chuck 401 having a structurewherein the chuck positive electrostatic layer 2 and the chuck negativeelectrostatic layer 3 are arranged between the ceramic substrate 1 andthe ceramic dielectric film 4 and the Peltier device 8 composed of thethermoelectric element 81 and the ceramic plate 82 is formed on thebottom surface of the ceramic substrate 1, as shown in FIG. 7.

A section of the edge of the electrostatic electrodes in theseelectrostatic chucks is in a peaked shape Therefore, a large chuck poweris induced. Cracks are not easily generated in the ceramic substrate.

As shown in FIGS. 3 through 7, in the present invention the chuckpositive electrostatic layer 2 and the chuck negative electrostaticlayer 3 are arranged between the ceramic substrate 1 and the ceramicdielectric film 4 and the resistance heating elements 5 or the metalline 7 is formed inside the ceramic substrate 1. Therefore, connectingunits (plated through holes) 16,17 are necessary for connecting theseunits to external terminals. The plated through holes 16,17 are made byfilling with a high melting point metal such as tungsten paste ormolybdenum paste, or a conductive ceramic such as tungsten carbide ormolybdenum carbide.

The diameter of the connecting units (plated through holes) 16,17 isdesirably from 0.1 to 10 mm. This is because disconnection can beprevented and further cracks or strains can be prevented.

The plated through holes are used as connecting pads to connect externalterminals 6,18 (see FIG. 8(d)).

The connecting thereof is performed with solder or brazing-filler. Asthe brazing-filler, brazing silver, brazing palladium, brazing aluminum,or brazing gold is used. Brazing gold is desirably Au—Ni alloy. Au—Nialloy is superior in adhesiveness to tungsten.

The ratio of Au/Ni is desirably [81.5 to 82.5 (% by weight)]/[18.5 to17.5 (% by weight)].

The thickness of the Au—Ni layer is desirably from 0.1 to 50 μm. This isbecause this range is a range sufficient for keeping connection. IfAu—Cu alloy is used at a high temperature of 500 to 1000° C. and at ahigh vacuum of 10⁻⁶ to 10⁻⁵ Pa, the Au—Cu alloy deteriorates. However,Au—Ni alloy does not cause such deterioration and is profitable. Whenthe total amount of the Au—Ni alloy is regarded as 100 parts by weight,the amount of impurities therein is desirably below 1 part by weight.

If necessary, in the present invention a thermocouple may be buried inthe bottomed hole in the ceramic substrate. This is because thethermocouple makes it possible to measure the temperature of theresistance heating element and, on the basis of the resultant data,voltage or electric current is changed so that the temperature can becontrolled.

The size of the connecting portions of metal lines of the thermocouplesis the same as the strand diameter of the respective metal lines ormore, and is preferably 0.5 mm or less. Such a structure makes thethermal capacity of the connecting portion small, and causes atemperature to be correctly and speedy converted to a current value. Forthis reason, temperature control ability is improved so that thetemperature distribution of the heated surface of the wafer becomessmall.

Examples of the thermocouple include K, R, B, S, E, J and T typethermocouples, described in JIS-C-1602 (1980).

FIG. 11 is a sectional view that schematically shows a supporting case41 into which the electrostatic chuck of the present invention, having astructure as described above, is fitted.

The electrostatic chuck 101 is fitted into the supporting case 41through a heat insulator 45. Coolant outlets 42 are made in thesupporting case 11, and a coolant is blown from a coolant inlet 44 andpasses outside an inhalation duct 43 through the coolant outlet 42. Bythe action of this coolant, the electrostatic chuck 101 can be cooled.

The following will describe one example of the process for producing anelectrostatic chuck as one example of the ceramic substrate of thepresent invention on the basis of sections shown in FIGS. 8(a) through(d).

-   (1) First, ceramic powder of a nitride ceramic, a carbide ceramic    and the like is mixed with a binder and a solvent to obtain a green    sheet 50.

As the ceramic powder, there may be used, for example, oxygen-containingaluminum nitride powder. If necessary, a sintering aid such as aluminaor sulfur may be added.

One or several green sheets 50′ laminated on the green sheet on which anelectrostatic electrode layer printed unit 51 that will be describedlater is formed are layers which will be a ceramic dielectric film 4;therefore, the sheets 50′ may have a composition different from that ofthe ceramic substrate if necessary.

Usually, the raw material of the ceramic dielectric film 4 and that ofthe ceramic substrate 1 are desirably the same. This is because theseare sintered under the same condition since these are sintered togetherin many cases. In the case that the raw materials are different, it isallowable that a ceramic substrate is firstly produced, an electrostaticelectrode layer is formed thereon and then a ceramic dielectric film isformed thereon.

As the binder, desirable is at least one selected from an acrylicbinder, ethylcellulose, butylcellusolve, and polyvinyl alcohol.

As the solvent, desirable is at least one selected from α-terpineol andglycol.

A paste obtained by mixing these is formed into a sheet form by thedoctor blade process. Thus, the green sheet 50 is obtained.

If necessary, a through hole into which a support pin of a silicon waferis inserted, a concave portion in which the thermocouple is buried maybe made in the green sheet 50, and further a through hole may be made ina portion where a plated through hole and the like is to be formed. Thethrough hole can be made by punching and the like.

The thickness of the green sheet is preferably from about 0.1 to 5 mm.

Next, a conductor containing paste is filled up into the through holesin the green sheet 50, to obtain plated through hole printed units53,54. Next, a conductor containing paste that will be electrostaticelectrode layers and resistance heating elements is printed on the greensheet 50.

The printing is performed to obtain a desired aspect ratio, consideringthe shrinkage ratio of the green sheet 50. In this way, electrostaticelectrode layer printed units 51 and resistance heating element layerprinted units 52 are obtained.

The printed units are formed by printing a conductor containing pastecontaining conductive ceramic or metal particles, and the like.

As the conductive ceramic particles contained in the conductorcontaining paste, carbide of tungsten or molybdenum is optimal. This isbecause they are not easily oxidized and their thermal conductivity isnot easily lowered.

As the metal particles, tungsten, molybdenum, platinum, nickel and thelike can be used.

The average particle diameters of the conductive ceramic particles andthe metal particles are preferably from 0.1 to 5 μm. This is because theconductor containing paste is not easily printed in either case thatthese particles are too large or too small.

As such a paste, the following conductor containing paste is optimal: aconductor containing paste prepared by mixing 85 to 97 parts by weightof the metal particles or the conductive ceramic particles; 1.5 to 10parts by weight of at least one binder selected from acrylic,ethylcellusolve, butylcellusolve and polyvinyl alcohol; 1.5 to 10 partsby weight of at least one solvent selected from α-terpineol, glycol,ethyl alcohol, and butanol.

Next, as shown in FIG. 8(a), the green sheet 50 having the printed units51, 52, 53 and 54 and the green sheet 50′ having no printed units aremade into a lamination. The reason why the green sheet 50′ having noprinted units at the side where the resistance heating elements areformed is deposited is that the following phenomenon is prevented: theend faces of the plated through holes are exposed and the end faces areoxidized at the time of the sintering for the formation of theresistance heating elements. If the sintering for the formation of theresistance heating elements is performed in the state that the end facesof the plated through holes are exposed, it is necessary to sputter ametal which is not easily oxidized, such as nickel. More preferably, theend faces may be covered with brazing gold of Au—Ni.

-   (2) Next, as shown in FIG. 8(b), the lamination is heated and    pressed to form a lamination of the green sheets. The conductor    containing paste layer is pressed at this time. Thus, in the case    that the formed conductor containing paste layer contains an    appropriate binder, a section of the edge thereof turns into a    peaked shape. The heating temperature of the lamination is    preferably from 50 to 300° C., and the pressure upon the pressing is    preferably from 20 to 200 kg/cm².

Thereafter, the green sheets and the conductor containing paste aresintered.

The temperature at the time of the sintering is preferably from 1000 to2000° C. and pressure at the time of the sintering is preferably from100 to 200 kgf/cm². The heating and the pressing are performed in theatmosphere of inert gas. As the inert gas, argon, nitrogen and the likecan be used. In this sintering step, plated through holes 16 and 17, andalso the chuck positive electrostatic layer 2, the chuck negativeelectrostatic layer 3, the resistance heating elements 5, the edge ofwhich is in a peaked shape, and so on can be formed.

-   (3) Next, as shown in FIG. 8(c), blind holes 35 and 36 for    connecting external terminals are made.

It is desirable that at least a part of inside walls of the blind holes35,36 is made conductive and the conductive part of the inside walls isconnected to the chuck positive electrostatic layer 2, the chucknegative electrostatic layer 3, the resistance heating elements 5 and soon.

-   (4) At last, as shown in FIG. 8(d), external terminals 6 and 18 are    fitted into the bottomed holes 35,36 through brazing gold. If    necessary, a bottomed hole may be made so that a thermocouple may be    buried therein.

As solder, an alloy such as silver-lead, lead-tin or bismuth-tin can beused. The thickness of the solder layer is desirably from 0.1 to 50 μm.This is because this range is a range sufficient for maintaining theconnection based on the solder.

In the above description, the electrostatic chuck 101 (see FIG. 3) isgiven as an example. In the case that the electrostatic chuck 201 (seeFIG. 5) is produced, it is advisable to produce a ceramic plate havingan electrostatic electrode layer first, print a conductor containingpaste on the bottom surface of this ceramic plate, sinter the resultant,form the resistance heating elements 25 and then form the metal coveringlayer 25 a by electroless plating or the like. In the case that theelectrostatic chuck 301 (see FIG. 6) is produced, it is advisable that ametal foil or a metal line is buried, as electrostatic electrodes orresistance heating elements, in ceramic powder and then the resultant issintered.

In the case that the electrostatic chuck 401 (see FIG. 7) is produced,it is advisable that a ceramic plate having an electrostatic electrodelayer is produced and then a Peltier device is jointed to the ceramicplate through a metallized metal layer.

The above-mentioned ceramic substrate functions as a wafer prober in thefollowing case: conductors are arranged on the surface of the ceramicsubstrate of the present invention and inside the ceramic substrate; theconductor layer on the surface is a chuck top conductor layer; and theinside conductor is either of a guard electrode or a ground electrode.

FIG. 12 is a sectional view that schematically shows one embodiment ofthe wafer prober of the present invention. FIG. 13 is a sectional viewtaken along A—A line in the wafer prober shown in FIG. 12.

In this wafer prober 501, grooves 67, in the form of concentric circlesas viewed from the above., are made in the surface of a disc-formceramic substrate 63. Moreover, suction holes 68 for sucking a siliconwafer are made in a part of the grooves 67. A chuck top conductor layer62 for connecting electrodes of the silicon wafer is formed, in acircular form, in the greater part of the ceramic plate 63 including thegrooves 67.

On the other hand, resistance heating elements 61 as shown in FIG. 1, inthe form of concentric circles as viewed from the above, are disposed onthe bottom surface of the ceramic substrate 63 to control thetemperature of the silicon wafer. External terminals (not illustrated)are connected and fixed to both ends of the resistance heating element61. Inside the ceramic substrate 63, a guard electrode 65 and a groundelectrode 66 (see FIG. 13), in the form of a lattice as viewed from theabove, are disposed. Since a section of the edge of the guard electrode65 and the ground electrode 66 is in a peaked shape, cracks are noteasily generated.

In the electrostatic chuck, the resistance heating elements 61 may beset inside the ceramic substrate 63. In this case, if a section of theedge of the resistance heating elements 61 is made into a peaked shape,cracks are not easily generated and a heat-accumulating phenomenon isnot easily generated.

The raw material of the guard electrode 65 and the ground electrode 66may be the same as that of the electrostatic electrode.

The thickness of the chuck top conductor layer 62 is desirably from 1 to20 μm. If the thickness is below 1 μm, its resistance is too high so asnot to function as an electrode. On the other hand, if the thickness isover 20 μm, the layer exfoliates easily by stress that the conductorhas.

As the chuck top conductor layer 62, there can be used, for example, atleast one metal selected from high melting point metals such as copper,titanium, chromium, nickel, noble metals (gold, silver, platinum and soon), tungsten and molybdenum.

According to the wafer prober having such a structure, a continuity testcan be performed by putting a silicon wafer on which integrated circuitsare formed, pushing a probe card having tester pins against the siliconwafer, and applying a voltage thereto while heating and cooling thewafer. In the case that a wafer prober is produced, for example, aceramic substrate wherein resistance heating elements are embedded isfirstly produced in the same manner as in the case of the electrostaticchuck. Thereafter, grooves are made on the surface of the ceramicsubstrate and subsequently the surface in which the grooves are made issubjected to sputtering, plating and so on, to form a metal layer.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will be described in more detailed hereinafter.

EXAMPLE 1 Production of an Electrostatic Chuck (See FIG. 3)

-   (1) The following paste was used to conduct formation by doctor    blade method to obtain a green sheet 0.47 μm in thickness: a paste    obtained by mixing 100 parts by weight of aluminum nitride powder    (made by Tokuyama Corp., average particle diameter: 1.1 μm) 4 parts    by weight of yttria (average particle diameter: 0.4 μm), 11.5 parts    by weight of an acrylic binder, 0.5 part by weight of a dispersant    and 53 parts by weight of mixed alcohols of 1-butanol and ethanol.-   (2) Next, this green sheet was dried at 80° C. for 5 hours, and    subsequently the following holes were made by punching: holes which    would be through holes through which semiconductor wafer supporting    pins 1.8 mm, 3.0 mm and 5.0 mm in diameter were inserted; and holes    which would be plated through holes for connecting external    terminals.-   (3) The following were mixed to prepare a conductor containing paste    A: 100 parts by weight of tungsten carbide particles having an    average particle diameter of 1 μm, 3.0 parts by weight of an acrylic    binder, 3.5 parts by weight of α-terpineol solvent, and 0.3 part by    weight of a dispersant.

The following were mixed to prepare a conductor containing paste B: 100parts by weight of tungsten particles having an average particlediameter of 3 μm, 1.9 parts by weight of an acrylic binder, 3.7 parts byweight of α-terpineol solvent, and 0.2 part by weight of a dispersant.

This conductor containing paste A was printed on the green sheet byscreen printing to form a conductor containing paste layer. The patternof the printing was made into a concentric pattern. Furthermore,conductor containing paste layers having an electrostatic electrodepattern shown in FIG. 4 were formed on other green sheets.

Moreover, the conductor containing paste B was filled into the throughholes for the plated through holes for connecting external terminals.

Thirty four green sheets 50′ on which no tungsten paste was printed werestacked on the upper side (heating surface) of the green sheet 50 thathad been subjected to the above-mentioned processing, and the samethirteen green sheets 50′ were stacked on the lower side of the greensheet 50. The green sheet 50 on which the conductor containing pastelayer having the electrostatic electrode pattern was printed was stackedthereon. Furthermore, two green sheets 50′ on which no tungsten pastewas printed were stacked thereon. The resultant was pressed at 130° C.and a pressure of 80 kg/cm² to form a lamination (FIG. 8(a)).

-   (4) Next, the resultant lamination was degreased at 600° C. in the    atmosphere of nitrogen gas for 5 hours and hot-pressed at 1890° C.    and a pressure of 150 kg/cm² for 3 hours to obtain an aluminum    nitride plate 3 mm in thickness. This was cut off into a disk of 230    mm in diameter to prepare a plate made of aluminum nitride and    having therein resistance heating elements 5 having a thickness of 6    μm and a width of 10 μm, a chuck positive electrostatic layer 2    having a thickness of 10 μm, and a chuck negative electrostatic    layer 3 having a thickness of 10 μm (FIG. 8(b)).-   (5) Next, the plate obtained in the (4) was polished with a diamond    grindstone. Subsequently a mask was put thereon, and bottomed holes    (diameter: 1.2 mm, and depth: 2.0 mm) for thermocouples were made in    the surface by blast treatment with SiC and the like.-   (6) Furthermore, portions corresponding to where the plated through    holes were made were hollowed out to make blind holes 35,36 (FIG.    8(c)). Brazing gold made of Ni—Au was heated and allowed to reflow    at 700° C. to connect external terminals 6, 18 made of Koval to the    blind holes 35,36 (FIG. 8(d)).

About the connection of the external terminals, a structure, wherein asupport of tungsten supports at three points, is desirable. This isbecause the reliability of the connection can be kept.

-   (7) Next, thermocouples for controlling temperature were buried in    the bottomed holes to finish the production of an electrostatic    chuck having the resistance heating elements.

The resultant ceramic substrate constituting the electrostatic chuck wascut in the manner that the cut section would contain the electrostaticelectrode layer. The section was observed with a scanning electronmicroscope (SEM). The results are shown in FIG. 14. As is evident fromFIG. 14, the section of the edge of the electrostatic electrode layerwas in a peaked shape.

COMPARATIVE EXAMPLE 1

Green sheets were formed to be a lamination in the same manner as inExample 1 except that it was carried out without heating and at thetemperature of 25° C. The observation thereof with the scanning electronmicroscope (500 magnifications) demonstrated that edges of sections ofboth the electrostatic electrodes and the resistance heating elementshad a face perpendicular to the wafer-processing surface.

(EXAMPLE 2) Production of a Wafer Prober 201 (See FIG. 12)

-   (1) The following paste was used to conduct formation by doctor    blade method to obtain a green sheet 0.47 μm in thickness: a paste    obtained by mixing 1000 parts by weight of aluminum nitride powder    (made by Tokuyama Corp., average particle diameter: 1.1 μm), 40    parts by weight of yttria (average particle diameter: 0.4 μm), and    530 parts by weight of mixed alcohol's of 1-butanol and ethanol.-   (2) Next, this green sheet was dried at 80° C. for 5 hours, and    subsequently through holes for plated through holes for connecting    external terminals to resistance heating elements were made by    punching.-   (3) The following were mixed to prepare a conductor containing paste    A: 100 parts by weight of tungsten carbide particles having an    average particle diameter of 1 μm, 3.0 parts by weight of an acrylic    binder, 3.5 parts by weight of α-terpineol solvent, and 0.3 part by    weight of a dispersant.

The following were mixed to prepare a conductor containing paste B: 100parts by weight of tungsten particles having an average particlediameter of 3 μm, 1.9 parts by weight of an acrylic binder, 3. 7 partsby weight of a-terpineol solvent, and 0.2 part by weight of adispersant.

Next, a printed unit for a guard electrode and a printed unit for aground electrode in a lattice form were printed on the green sheet byscreen sprinting using this conductor containing paste A.

Moreover, the conductor containing paste B was filled into the throughholes for the plated through holes for connecting external terminals.

Fifty green sheets on which the paste was printed and no paste wasprinted were stacked. The resultant was pressed at 130° C. and apressure of 80 kg/cm² to form a lamination.

-   (4) Next, this lamination was degreased at 600° C. in the atmosphere    of nitrogen gas for 5 hours and hot-pressed at 1890° C. and a    pressure of 150 kg/cm² for 3 hours to obtain a aluminum nitride    plate 3 mm in thickness. The resultant plate was cut off into a disk    of 300 mm in diameter to prepare a ceramic plate. Regarding the size    of plated through holes 17, their diameter was 0.2 mm and their    depth was 0.2 mm.

The thickness of a guard electrode 65 and a ground electrode 66 was 10μm. The position where the guard electrode 65 was formed were 1 mm apartfrom the wafer-putting surface. The position where the ground electrode66 was formed was 1.2 mm apart from the wafer-putting surface. A sectionof the edge of the guard electrode 65 and the ground electrode 66 was ina peaked shape. The length of one side of each non-conductor formingarea 66 a in the guard electrode 65 and the ground electrode 66 was 0.5mm.

-   (5) Next, the plate obtained in the (4) was polished with a diamond    grindstone. Subsequently a mask was put thereon, and concaves for    thermocouples, and grooves 67 (width: 0.5 mm, and depth: 0.5 mm) for    adsorbing a wafer were made in the surface by blast treatment with    SiC and the like.-   (6) Furthermore, a layer for forming resistance heating elements 61    was printed on the surface opposite to the wafer-putting surface. A    conductor containing paste was used for the printing. The used    conductor containing paste was Solvest PS603D made of Tokuriki    Kagaku Kenkyu-zyo, which is used to form plated through holes in    printed circuit boards. This conductor containing paste was a    silver/lead paste, and contained 7.5 parts by weight of metal oxides    consisting of lead oxide, zinc oxide, silica, boron oxide and    alumina (the weight ratio thereof was 5/55/10/25/5) per 100 parts by    weight of silver.

The shape of the silver was scaly and had an average particle diameterof 4.5 μm.

-   (7) The ceramic substrate 63 on which the conductor containing paste    was printed was heated and fired at 780° C. to sinter and bake    silver and lead in the conductor containing paste on the ceramic    substrate 63. The heater plate was immersed in a bath for    electroless nickel plating consisting an aqueous solution containing    30 g/L of nickel sulfate, 30 g/L of boric acid, 30 g/L of ammonium    chloride, and 60 g/L of a Rochelle salt, to precipitate a nickel    layer (not illustrated) having a thickness of 1 μm and a boron    content of 1% or less by weight on the surface of the resistance    heating elements 61 made of the sintered silver Thereafter, the    heater plate was annealed at 120° C. for 3 hours.

The resistance heating elements made of the sintered silver had athickness of 5 μm, a width of 2.4 mm and a area resistivity of 7.7 mΩ/□.

-   (8) By sputtering, a titanium layer, a molybdenum layer and a nickel    layer were successively formed on the surface in which the grooves    67 were made. The used equipment for the sputtering was SV-4540 made    of ULVAC Japan, Ltd. About conditions for the sputtering, air    pressure was 0.6 Pa, temperature was 100° C. and electric power was    200 W. Sputtering time was within the range of 30 seconds to 1    minute, and was adjusted dependently on the respective metals.

About the thickness of the resultant films, an image from a fluorescentX-ray analyzer demonstrated that the thickness of the titanium layer was0.3 μm, that of the molybdenum layer was 2 μm and that of the nickellayer was 1 μm.

-   (9) The ceramic substrate 63 obtained in the above (8) was immersed    in a bath for electroless nickel plating consisting an aqueous    solution containing of 30 g/L of nickel sulfate, 30 g/L of boric    acid, 30 g/L of ammonium chloride, and 60 g/L of a Rochelle salt to    precipitate a nickel layer having a thickness of 7 μm and a boron    content of 1% or less by weight on the surface of the metal layer    made by the sputtering. Thereafter, the resultant ceramic substrate    was annealed at 120° C. for 3 hours.

No electric current was not allowed to pass on the surface of theresistance heating elements, and the surface was not coated with anyelectroplating nickel.

The ceramic substrate was immersed in an electroless gold platingsolution containing 2 g/L of potassium gold cyanide, 75 g/L of ammoniumchloride, 50 g/L of sodium citrate, and 10 g/L of sodium hypophosphiteat 93° C. for 1 minute to form a gold plating layer 1 μm in thickness onthe nickel plating layer.

-   (10) Air suction holes 68 piercing the back surface from the grooves    67 were made by drilling, and then blind holes (not illustrated) for    exposing plated through holes 16 were made. Brazing gold made of    Ni—Au (Au: 81.5% by weight, Ni: 18.4% by weight, and impurities:    0.1% by weight) was heated and allowed to reflow at 970° C. to    connect external terminals made of koval to the blind holes.    External terminals made of koval were also attached through a solder    (tin: 90% by weight, and lead: 10% by weight) on the resistance    heating elements.-   (11) Next, thermocouples for controlling temperature were buried in    the concaves to obtain a wafer prober heater 201.

In the ceramic substrate, its pore diameter of the maximum pore was 2μm, and its porosity was 1%. The temperature of the ceramic substratewas raised to 200° C. Even if 200 V was applied thereto, no dielectricbreakdown was caused. Its warp amount was 1 μm or less, which was good.

COMPARATIVE EXAMPLE 2

Green sheets were formed to be a lamination in the same manner as inExample 2 except that it was carried out without heating and at thetemperature of 25° C. The observation thereof with the scanning electronmicroscope (500 magnifications) demonstrated that edges of sections ofboth the guard electrode and the ground electrode had a faceperpendicular to the wafer-processing surface.

EXAMPLE 3 Alumina Hot Plate (See FIGS. 1 and 2)

-   (1) The following paste was used to conduct formation by doctor    blade method to obtain green sheets 0.47 μm in thickness: a paste    obtained by mixing alumina: 93 parts by weight, SiO₂: 5 parts by    weight, CaO: 0.5 part by weight, MgO: 0.5 part by weight, TiO₂: 0.5    part by weight, an acrylic binder: 11.5 parts by weight, 0.5 part by    weight of a dispersant and 53 parts by weight of mixed alcohols of    1-butanol and ethanol.-   (2) Next, these green sheets were dried at 80° C. for 5 hours, and    subsequently the following holes were made in the green sheets for    which they are necessary by punching: holes which would be through    holes through which semiconductor wafer supporting pins having    diameter of 1.8 mm, 3.0 mm and 5.0 mm were inserted; and holes which    would be plated through holes for connecting external terminals.-   (3) The following were mixed to prepare a conductor containing paste    B: 100 parts by weight of tungsten particles having an average    particle diameter of 3 μm, 1.9 parts by weight of an acrylic binder,    3.7 parts by weight of α-terpineol solvent, and 0.2 part by weight    of a dispersant.

This conductor containing paste B was printed on the green sheet byscreen printing to form a conductor containing paste layer. The patternof the printing was made into a concentric pattern.

-   (4) Moreover, the conductor containing paste B was filled into the    through holes for the plated through holes for connecting external    terminals.

Furthermore, 34 to 60 sheets on which no tungsten paste was printed werestacked on the upper surface (heating surface) of the green sheet onwhich the pattern of the resistance heating elements was formed, and thesame 13 to 30 green sheets were stacked on the lower side of the greensheet. This was pressed at 130° C. and a pressure of 80 kg/cm² to form alamination.

-   (5) Next, the resultant lamination was degreased at 600° C. in the    atmosphere of air for 5 hours and hot-pressed at 1600° C. and a    pressure of 150 kg/cm² for 2 hours to obtain an aluminum nitride    plate 3 mm in thickness. This was made to a substrate 280 mm in    diameter and 19 mm in thickness by changing working conditions and    polishing conditions.

A plate which was made of alumina and had therein resistance heatingelements 5 having a thickness of 6 μm and a width of 10 mm was prepared.A section of the edge of the resistance heating elements 5 was in apeaked shape.

-   (6) Next, the plate obtained in the (3) was polished with a diamond    grindstone. Subsequently a mask was put thereon, and bottomed holes    (diameter: 1.2 mm, and depth: 2.0 mm) for thermocouples were made in    the surface by blast treatment with SiC and the like.-   (7) Furthermore, portions corresponding to where the plated through    holes were made were hollowed out to make blind holes. Brazing gold    made of Ni—Au was heated and allowed to reflow at 700° C. to connect    external terminals made of koval to the blind holes.

About the connection of the external terminals, a structure, wherein asupport of tungsten supports at three points, is desirable. This isbecause the reliability of the connection can be kept.

-   (8) Next, thermocouples for controlling temperature were buried in    the bottomed holes to finish the production of a hot plate having    the resistance heating elements.

COMPARATIVE EXAMPLE 3

Green sheets were formed to be a lamination in the same manner as inExample 3 except that it was carried out without heating and at thetemperature of 25° C. The observation thereof with the scanning electronmicroscope (500 magnifications) demonstrated that edges of sections ofthe resistance heating elements had a face perpendicular to thewafer-processing surface.

EXAMPLE 4 Hot Plate Made of Aluminum Nitride (See FIGS. 1 and 2)

-   (1) The following paste was used to conduct formation by doctor    blade method to obtain a green sheet 0.47 μm in thickness: a paste    obtained by mixing 100 parts by weight of aluminum nitride powder    (made by Tokuyama Corp., average particle diameter: 1.1 μm), 4 parts    by weight of yttria (average particle diameter: 0.4 μm), 11.5 parts    by weight of an acrylic binder, 0.5 part by weight of a dispersant,    8 parts by weight of an acrylic resin binder (made by Kyoeisyha    Chemical Co., Ltd., trade name: KC-600, and acid value: 17 KOHmg/g)    and 53 parts by weight of mixed alcohols of 1-butanol and ethanol.-   (2) Next, this green sheet was dried at 80° C. for 5 hours, and    subsequently the following holes were made by punching: holes which    would be through holes through which semiconductor wafer supporting    pins with a diameter of 1.8 mm, 3.0 mm and 5.0 mm were inserted; and    holes which would be plated through holes for connecting external    terminals.-   (3) The following were mixed to prepare a conductor containing paste    A: 100 parts by weight of tungsten carbide particles having an    average particle diameter of 1 μm, 3.0 parts by weight of an acrylic    binder, 3.5 parts by weight of α-terpineol solvent, and 0.3 part by    weight of a dispersant.

The following were mixed to prepare a conductor containing paste B: 100parts by weight of tungsten particles having an average particlediameter of 3 μm, 1.9 parts by weight of an acrylic binder, 3.7 parts byweight of α-terpineol solvent, and 0.2 part by weight of a dispersant.

This conductor containing paste A was printed on the green sheet byscreen printing to form a conductor containing paste layer. The patternof the printing was made into a concentric pattern. Furthermore,conductor containing paste layers having an electrostatic electrodepattern shown in FIG. 10 were formed on other green sheets.

Moreover, the conductor containing paste B was filled into the throughholes for the plated through holes for connecting external terminals.

At 130° C. and a pressure of 80 kg/cm², thirty seven green sheets onwhich no tungsten paste was printed were stacked on the upper side(heating surface) of the green sheet that had went through theabove-mentioned processing, and simultaneously the same thirteen greensheets were stacked on the lower side of the green sheet.

-   (4) Next, the resultant lamination was degreased at 600° C. in the    atmosphere of nitrogen gas for 1 hour and hot-pressed at 1890° C.    and a pressure of 150 kg/cm² for 3 hours to obtain an aluminum    nitride plate 3 mm in thickness, which contained 810 ppm of carbon.    This was cut into a disk of 230 mm in diameter to prepare a plate    having therein resistance heating elements having a thickness of 6    μm and a width of 10 mm and electrostatic electrodes. A section of    the edge of the electrostatic electrodes was in a peaked shape.-   (5) Next, the plate obtained in the (4) was polished with a diamond    grindstone. Subsequently a mask was put thereon, and bottomed holes    (diameter: 1.2 mm, and depth: 2.0 mm) for thermocouples were made in    the surface by blast treatment with SiC and the like.-   (6) Furthermore, portions corresponding to where the plated through    holes were made were hollowed out to make concaves. Brazing gold    made of Ni—Au was heated and allowed to reflow at 700° C. to connect    external terminals made of koval to the concaves.

About the connection of the external terminals, a structure, wherein asupport of tungsten supports at three points, is desirable. This isbecause the reliability of the connection can be kept.

-   (7) Next, thermocouples for controlling temperature were buried in    the bottomed holes to finish the production of a ceramic heater (hot    plate).

COMPARATIVE EXAMPLE 4

Green sheets were formed to be a lamination in the same manner as inExample 4 except that it was carried out without heating and at thetemperature of 25° C. The observation thereof with the scanning electronmicroscope (500 magnifications) demonstrated that edges of sections ofthe resistance heating elements had a face perpendicular to thewafer-processing surface.

Evaluation Method

(1) Heat Uniformity

A thermo viewer (IR-162012-0012, made by Japan Datum Company) was usedto measure temperatures at respective points on the wafer-puttingsurface of the ceramic substrates to obtain a temperature differencebetween the highest temperature and the lowest temperature.

(2) Adsorption Power

The temperature of the ceramic substrates was raised to 450° C. tomeasure the adsorption power thereof with a load cell (Autograph AGS-50,made by Shimadzu Corp.).

(3) Thermal Shock Resistance

The temperature of the ceramic substrates was raised to 200° C. and thenthe ceramic substrates were thrown into water, and it was examinedwhether cracks were generated or not. In Table 1, ◯ indicates that nocrack was generated and the ceramic substrate had thermal shockresistance, and x indicates that cracks were generated and the ceramicsubstrate had no thermal shock resistance.

(4) Leakage Current

A voltage of 1 kV was applied between conductors that were supposed tobe insulated in the ceramic substrates, and then a leakage current at300° C. was measured using a breakdown voltage tester (TOS-5051, made byKikusui Electronics Corp.) and an Ultrahigh Resistor (R8340, made byAdvantest Corp.).

TABLE 1 Heat Adsorption uniformity power Thermal shock Leakage (° C.)(g/cm²) resistance current (mA) Example 1 4 1000 ◯ 4 Example 2 5 — ◯ 3Example 3 10 — ◯ 3 Example 4 4 — ◯ 4 Comparative 8  800 X 8 example 1Comparative 10 — X 8 example 2 Comparative 20 — X 6 example 3Comparative 8 — X 9 example 4

As is clear from Table 1, the ceramic substrates according to Examples 1to 4 were superior in both heat uniformity and thermal shock resistance.As is also clear from Example 1 and Comparative example 1, theelectrostatic chuck according to Example 1 had a large chuck power.

Industrial Applicability

As described above, the ceramic substrate of the present invention issuperior in heat uniformity and thermal shock resistance. In the casethat the ceramic substrate is used as an electrostatic chuck, its chuckpower is large.

1. A ceramic heater comprising a ceramic substrate and a conductor layerformed inside said ceramic substrate, wherein a section of an edge ofthe conductor layer has a peaked shape in a cross-section orientednormal to the plane of the conductor layer, and the thickness of theceramic substrate is 25 mm or less.
 2. A hot plate comprising theceramic heater of claim
 1. 3. The ceramic heater of claim 1, wherein thepeak shaped section of the edge of the conductor layer has a width of0.1 to 200 μm.
 4. The ceramic heater of claim 1, wherein the peak shapedsection of the edge of the conductor layer has a width of 5 to 100 μm.5. The ceramic heater of claim 1, wherein the peak shaped section of theedge of the conductor layer has a radius of curvature of 0.5 to 500 μm.6. The ceramic heater of claim 1, wherein the ceramic substrate hasporosity of 5% or less.
 7. The ceramic heater of claim 1, wherein theceramic substrate has a maximum pore diameter of 50 μm or less.
 8. Theceramic heater of claim 1, wherein the ceramic substrate comprises atleast one ceramic selected from the group consisting of nitrideceramics, carbide ceramics, oxide ceramics, and mixtures thereof.
 9. Theceramic heater of claim 8, wherein the ceramic substrate comprises atleast one nitride ceramic selected from the group consisting of aluminumnitride, silicon nitride, boron nitride, and mixtures thereof.
 10. Theceramic heater of claim 8, wherein the ceramic substrate comprises atleast one carbide ceramic selected from the group consisting of siliconcarbide, zirconium carbide, tantalum carbide, tungsten carbide, andmixtures thereof.
 11. The ceramic heater of claim 8, wherein the ceramicsubstrate comprises at least one oxide ceramic selected from the groupconsisting of alumina, zirconia, cordiente, mullite, and mixturesthereof.
 12. The ceramic heater of claim 8, wherein the ceramicsubstrate further comprises 0.1 to 5% by weight of oxygen.
 13. Theceramic heater of claim 12, wherein the ceramic substrate furthercomprises at least one of yttria, alumina, rubidium oxide, lithiumoxide, and calcium oxide.
 14. The ceramic heater of claim 8, wherein theceramic substrate further comprises 5 to 5000 ppm of carbon.
 15. Aprocess for making a ceramic heater, the process comprising sintering alaminate comprising a conductor layer between a first ceramic greensheet and at least one additional ceramic green sheet; and producing theceramic heater of claim 1.